The present invention is related to printed circuit boards and electromagnetic interference (EMI). In particular, the present invention is related to printed circuit boards with shielding against electromagnetic interference.
Printed circuit boards (PCB""s), including rigid, flexible, single, double and multilayer boards have found important uses in the semi-conductor and electronic industry. A PCB typically contains metallized layers of electrical circuitry or traces separated by dielectric material. Interconnections are provided for electrical connection between the metallized layers. In certain applications, such as audio-amplifiers, microphones and mobile telephones, the electrical circuitry of the PCB generates an electromagnetic field which interfere with the normal operation of other circuitries in the vicinity.
One conventional method of reducing EMI in circuitry embedded in PCB""s is to provide metal shield on the surfaces of the board. This shield may be one or more metal plates mounted around the PCB, or a layer of metallic coating plated directly onto the surface of the PCB. Another prior art method involves the plating of metallic traces internally within the inner layers of the PCB adjacent the prescribed trace or traces for which EMI shielding is needed.
FIGS. 1A to 1D show the traces and structures found in the prior art PCB for internal EMI shielding. A simple six-layer PCB is shown with a metal trace 20 in layer L1-3 and another metal trace 22 in layer L1-4. These two traces require EMI shielding. For ease of illustration, only the traces to be shielded and the relevant shielding structures are shown in layers L1-2 to L1-5. Trace 20 is interconnected to trace 22 through metal-coated via 24. Two ground shields 26 and 28 are also plated in layer L1-3 on the two opposing sides of trace 20. Ground shield 21, connected to traces 26 and 28 in layer L1-3, is provided for top shielding of trace 22. Two other grounded traces 30 and 32 are plated in L1-4 on the two adjacent sides of trace 22. They are connected to trace 31, which act as the bottom shield for trace 20. Ground shield 33 is provided in layer L1-2 directly above metal trace 20 to function as top shielding against EMI. Ground shield 35 is provided in layer L1-5 directly below metal trace 22 as bottom shield therefor. Between metallic layers L1-2, L1-3, L1-4 and L1-5 are dielectric layers, denoted as L1-axb where a and b indicate the layers directly above and below it respectively. To provide partial EMI shielding at the dielectric layers, conventional PCB boards are provided with vias 34a and 34b having a coat of metal 36 thereon.
It is clear from the above description of the prior art that the shielding provided within the PCB does not provide good EMI shielding, as there are multiple gaps 38 between vias 34a and 34b that do not provide shielding.
It is therefore an object of the present invention to provide a PCB board with improved internal EMI shielding.
Accordingly, the present invention provides PCB boards with in-board EMI shielding in which a prescribed electrical circuitry within a PCB is shielded from EMI by fabricating a uniform metal shielding enclosure therearound. The shielded circuitry may be a single metal trace, a group of interconnected traces in the same or different layers of the same PCB, or a complete electrical circuitry spanning many different layers and locations within the same PCB. Using the same technology, groups of electrical circuitry that require isolation because of EMI may be fabricated together in proximity, with each group shielded from the EMI of the others. The metal shield contains a top shield in a layer above the circuitry, a bottom shield in a layer below the circuitry, and side shields adjacent the circuitry.
According to one embodiment of the present invention, the side shields are trenches with metal coating provided adjacent the prescribed circuitry. The side trenches connect the top shield and the bottom shield without gaps. Openings of the uniform metal shield are provided at the sites where the circuitry connects to another circuitry within the same PCB board or with the exterior.
In another embodiment, the side shields are solid walls provided adjacent the prescribed circuitry. The walls are built using electroplating and/or electroless plating as described in detail below. As with the metal coated trenches, the solid walls connect the top shield and the bottom shield to form a uniform enclosure around the prescribed circuitry, except for openings where the prescribed circuitry connects to another circuitry within the same PCB board or with the exterior. The present invention not only provides complete shielding of a circuitry, but also gives substantial internal space savings.
As used herein, a uniform metallic shield refers to the a complete shield absent any non-metallic gaps within the side shields that connect the top shield and the bottom shield.